It happens in Eversong Forest, one of the first quests you get there actually. Their reasoning was that they were trying to help their allies (the Night Elves) gain territory on land that they saw as open territory since the Scourge had devastated it. Like any good military, they were aware of the weakened Blood Elf presence and were taking steps to ensure that the Belves couldn't threaten their claim on the territory.

So, where's there mentions of anything from your fantasy in official text?

Also both Garithos and Night Elves (as mentioned above) happened to be quite right in their appraisal of Kael'thas and elves that followed him, so we have:

Mad Scientists that was doing horrible experiments with full support from Sylvanas, who finally gone rogue and used Sylvanas-authorized horrible weapon in the way she didn't meant too (she wanted to use it only on living)

vs.

War veteran that didn't trust magic junkie haughty prince and still gave him several quite critical for his own campaign tasks, before finally admitting that this elf is just too dangerous, which proved absolutely correct in the end.

Go ahead, tell me that hordie is just rogue without backup and Alliance guy is horrible criminal. I wouldn't mind a laugh now.

Garithos was given the title "Grand Marshal" because he was the only military commander left in Northern EK. The dude never answered to anybody, he died before that could happen. I'm sure if he did he'd have probably been stripped of his title for his actions.

We know this is very true. My point was more that the Horde made it very, very clear that Putress was acting of his own accord and authority. To my knowledge, the Alliance has never ostracized the late Othmar Garithos like that, from either ignorance (it's not like there were many survivors to tell the tale) or otherwise.

How can we expect the blood elves (at least the ones he tried to execute) to have made that distinction when not even the Alliance did?

Who knows, maybe things would have gone down differently if that dwarven regiment that despised him had made it out of Lordaeron alive.

To my knowledge, the Alliance has never ostracized the late Othmar Garithos like that, from either ignorance (it's not like there were many survivors to tell the tale) or otherwise.

I think it was simply a case of Blizzard either forgetting or not bothering with it, he's simply not mentioned at all which I feel is probably the biggest reason I feel it's mostly likely the former or latter reason are the reason for it being case.
I'm sure somebody could tweet Metzen about it. In fact I might do that now.

We know this is very true. My point was more that the Horde made it very, very clear

Mind providing links or screenshots? I didn't play Horde until this quest chain was removed, so I'm quite interested in this. Quick look at http://www.wowpedia.org/Quest:The_Ba...ercity_(Horde) shows that Lich Bitch is only worried about her personal little kingdom.

Lady Sylvanas Windrunner says: What have they done to my beautiful city!
Lady Sylvanas Windrunner says: The only redemption for the traitors responsible for this will be an agonizing death. My vengeance will be swift and without mercy!

Yes, same "beautiful city" in which they kill, torture and mind control humans up to this day.

The general consensus (edit: amongst players, not in-game) is that the Horde was not responsible for the actions of the Forsaken at the Wrathgate, yet the Alliance as a whole is still held accountable for the actions against the blood elves (in Warcraft 3!).

Surely they're either both responsible, or neither of them are.

What exactly is the difference?

Perception of accountability and reality of accountability are two different things. Look at Gulf War 2 after 9/11. Look at current job markets and economies. True assumption of responsibility isn't a black and white matter. You can be partially responsible for something. There are no real answers to your questions.

You can argue the Horde was responsible for the Wrathgate Forsaken simply because they allowed this splinter cell to form and act without taking action against them earlier. You can argue they weren't responsible since it was a splinter cell that acted against both sides. Both arguments are reasonable.

Is a soldier responsible for the choices of his generals?

The common argument that the Forsaken used a plague that killed both sides and this would be unwanted or unsanctioned is flawed when it comes to the idea of accountability, as it assumes that intentions are more important than results. Just because you didn't desire an end result doesn't mean you aren't accountable for it.

Basically, the Horde should be accountable for the actions fo the Forsaken at Wrathgate, but is percieved to be unaccountable for those same actions. The perception among the player base doesn't match the reality. This is common to the human condition.

I think it was simply a case of Blizzard either forgetting or not bothering with it, he's simply not mentioned at all which I feel is probably the biggest reason I feel it's mostly likely the former or latter reason are the reason for it being case.
I'm sure somebody could tweet Metzen about it. In fact I might do that now.

That's actually a good point. You'd think with all the Garithos-related backstory we got last time, someone would have mentioned this to cdev. >.<

Thrall says: Kor'kron, stand down!
Thrall says: Jaina...
Lady Jaina Proudmoore says: Thrall, what has happened? The King is preparing for war...
Thrall says: Jaina, what happened at the Wrathgate. It was a betrayal from within...
Lady Sylvanas Windrunner says: Lady Proudmoore, the Warchief speaks the truth. This subterfuge was set in motion by Varimathras and Grand Apothecary Putress. It was not the Horde's doing.
Lady Sylvanas Windrunner says: As the combined Horde and Alliance forces begain their assault upon the Wrathgate, an uprising broke out in the Undercity. Varimathras and hordes of his demonic bretheren attacked. Hundreds of my people were slain in the coup. I barely managed to escape with my life.
Thrall says: The Horde has lost the Undercity.
Thrall says: We now prepare to lay siege to the city and bring in the perpetrators of the unforgivable crime to justice.
Thrall says: If we are forced into a conflict, the Lich King will destroy our divided forces in Northrend.
Thrall says: We will make this right, Jaina. Tell your king all that you have learned here.
Lady Jaina Proudmoore says: I will deliver this information to King Wrynn, Thrall, but...
Lady Jaina Proudmoore says: Bolvar was like a brother to him. In the King's absence, Bolvar kept the Alliance united. He found strength for our people in our darkest hours. He watched over Anduin, raising him as his own.
Lady Jaina Proudmoore says: I fear that the rage will consume him, Thrall. I remain hopeful that reason will prevail, but we must prepare for the worst... for war.
Lady Jaina Proudmoore says: Farewell, Warchief. I pray that the next time we meet it will be as allies.

I don't think he could have made the Horde's opinion on Putress much clearer there.

Basically, the Horde should be accountable for the actions fo the Forsaken at Wrathgate, but is percieved to be unaccountable for those same actions. The perception among the player base doesn't match the reality. This is common to the human condition.

Wowpedia:

Putress was an important figure during the second Scourge Invasion, researching the zombie infestation from an apothecary camp in Shattrath City. Eventually, he began sending Horde members back to their capital cities with severed Scourge heads containing both the plague and an anti-plague, believing them to hold the secret of a cure. As he expected, his research did eventually lead to a cure, as well as his promotion to Grand Apothecary. When Warchief Thrall called for a meeting with other Horde leaders to discuss how to deal with the Scourge, Sylvanas Windrunner offered Putress's services to Thrall citing that his knowledge of the plague would make him a valuable asset in the campaign against the Lich King.

How very irresponsible of Lady Sylvanas to not trust a guy who had been a hero before. She should be checking up on him to make sure he wasn't utilizing the Plague that had saved lives to kill innocents.

More for you:

The Forsaken's aim is to create a new plague that would annihilate the Scourge itself, and ultimately, during the Northrend campaign, this is successful. However, the Forsaken's plague was usurped by the treacherous Grand Apothecary Putress, who proceeded to unleash it upon the Scourge, the Alliance and even the Horde. As such, the Forsaken's allies have deemed it unethical to be used again at its full potency.

Remember that the development of that there plague was made for Scourge only. It was backed by everyone to be scourge only. It was usurped by a guy who was previously a hero who had allied with a demon who was still loyal to Sargeras, but we all know how much the demons can turn good men who love their people into horrible pieces of crap.

Remember that the development of that there plague was made for Scourge only

To be fair, Sylvanas says (to herself) in Rise of the Lich King that it's meant for humanity too, as they're the race that "spawned" Arthas. She was testing the early stages of the plague on both undead and living humans.

To be fair, Sylvanas says (to herself) in Rise of the Lich King that it's meant for humanity too, as they're the race that "spawned" Arthas. She was testing the early stages of the plague on both undead and living humans.

True, and a whole lot of the early quests for Forsaken (starting zone in vanilla, I haven't done it since Cata) did some testing on living creatures. But:

The Forsaken Blight, combined with a reactive agent, is powerful enough to turn its victims (or at least the undead ones) into shapeless piles of goo. In the quests H [72] The Forsaken Blight and You: How Not to Die and H [72] Emerald Dragon Tears, Horde players are sent to find said reactive agent (the ectoplasmic residue from the ghosts of Arthas' troops on the Forgotten Shore) and a vessel to contain them (tears of green dragons from the Emerald Dragonshrine). Before delivering the plague to Doctor Sintar Malefious in Agmar's Hammer, the player must drive a Forsaken "blight spreader" catapult from Venomspite to an area of the Carrion Fields outside of Wintergarde Keep, right in the shadow of Naxxramas, to test this new blight on the Scourge (H [72] Spread the Good Word). It appears that the forces of the Scarlet Onslaught are immune to this plague, as indicated in the failed test against an Onslaught Prisoner in H [72] Blighted Last Rites.

As far as everyone was aware, by the time the plague was thrown at Angra'thar, the living were immune to it.